The present invention is directed towards a method and a particularly advantageous drive arrangement for regulating the individual drives of an arcuately shaped multi-roller continuous strand casting machine for metal, particularly steel, in which at least some of the drives powered by motor or by generator are regulated torque-dependently on the others, which creates compressive forces to the direction of the cast strand in the cross section of the strand.
A regulating method of this type is used in strand casting for the purpose of "compression casting". "Compression casting" refers to the compressive forces created in the direction of motion of the cast strand in the strand cross section. The coordination of the driving forces is to prevent an excessive tensional stress and/or bending stress during the cooling stages of the casting metal by creating, in each instance, counter-directed forces to the direction of travel the strand by means of the rolls which convey the casting strand.
It is necessary to create compressive forces in the strand cross section to the direction of motion of the casting strand by means of the conveyor means of the arcuately shaped multi-roller continuous strand casting machine in order to minimize the maximal stresses occurring in the area of the bending point. These compressive forces are absorbed, as negative tensional components, by the total tension, thereby relieving the tensionally stressed strand shell. The entire deformation of the strand shell is to be compared with the critical expansion value of the work material employed in order to reveal what the stress permissible is on the casting material during its cooling stage. The tension as well as the expansion of the strand shell constitute criteria in calculating the total stress. Also, the plastic and elastic behavior of the strand shell is to be taken into consideration in the given high temperatures.
It is difficult to calculate the stress on the strand shell because it is time dependent. The calculations may be based, within the alignment area, on interior deformation because of bulging; on tensile forces because of alignment; and on interior deformation because of roller striking and because of an uneven alignment of the roller track, as well as on the interior deformation by surface pressure of the rollers (hertz's pressure). The enumerated causes of stress may not simply be added together because of individually time-dependent factors.
From observations and re-calculations, and from the behavior of the steels in laboratory tests it is possible to estimate the progress of the strand-shell expansion in the area of solid/liquid interface, depending on time and temperature.
The factors thereby determined lead to the recognition that expansion factors above the predetermined limit cause increased damage in the work-material structure. Large shifts of the forming and solidifying crystals cause intercrystaline cracks which, because of further heavy stress, cannot close homogenously anymore so that liquid casting metal will fill the crack, thereby causing the subsequent crystalization to occur under changed chemical and physical conditions. The crack material, therefore, displays other properties than the work material surrounding it. The interior cracks cause some weak spots in the work material, which are to be classified according to product use, so that, in the end, types of material must be sorted out according to their intended requirements.